EP 0 516 538, discloses a writing instrument of that type whose buffer reservoir, which is made of an open-pore hydrophobic material, is made up of at least one compact block that fits snugly around the feed tip and that is coaxial therewith. Preferably, the porous hydrophobic material is based on microbeads or micro-spheres made, for example, of polypropylene.
The buffer reservoir disclosed in EP 0 516 538 is satisfactory from a functional point of view, i.e. technically it fulfills the functions of absorbing the surplus ink arriving in the feed tip when the pressure in the ink reservoir varies, and of subsequently returning the surplus ink to the feed tip. Unfortunately, the effectiveness of the buffer reservoir decreases radially going away from the zone in which the buffer reservoir is in contact with the feed tip. In other words, the ink diffuses easily from the feed tip into the buffer reservoir in a zone that is relatively close to the feed tip, which zone extends over a distance in the range 2 mm to 4 mm depending on the case. The greater the distance from the feed tip, the more difficult ink diffusion becomes, and it becomes reduced to zero at a distance approximately in the range 6 mm to 7 mm.
Given that the function of the buffer reservoir is to be capable of absorbing surplus ink due to a variation in the pressure in the ink reservoir, it will be understood that the working volume of the buffer reservoir is a function of the maximum volume of surplus ink that might result from such a variation. The volume of surplus ink is a function, in particular, of the volume of air contained in the liquid-ink reservoir, since it is generally an increase in the temperature of the air contained in said liquid-ink reservoir that causes the pressure variation which results in the surplus ink in the feed tip. The smaller the amount of ink remaining in the reservoir, the larger the amount of air contained therein, and the larger the amount of surplus ink for the same increase in temperature.
The problem that has been observed currently leads either to a buffer reservoir being provided that is very long, i.e. that has a large surface area over which it is in contact with the feed tip, or to the capacity of the liquid-ink reservoir being limited.